Ceramic Material and Manufacturing Methods



 Engineering Ceramics and Manufacturing Methods

The ceramic materials which might be used in place of other engineering materials such as metals, wools or plastics are termed as engineering ceramics. These materials withstand high temperatures, resist greater pressures have superior mechanical properties and can protect against corrosive chemicals.

BASIC RAW MATERIALS

The three main raw materials used for ceramic industry are clay, feldspar and sand.

1. CLAY

It is hydrated aluminum silicate. Some common clays are:

(i)           Kaolinite (AI2O3.2SiO2.2H2O0 )

(ii)         Montmorillortite (Mg, (Ca ) O, Al2O3.5SiO2.nH2O)

(iii)       Illite          (K2O, MgO, Al2O3. SiO2.H2O all in variable amounts).

2. FELDSPAR 

There are three common types of feldspar,:

(i)           Potash feldspar. (K2O. Al2O3. 6SiO2)

(ii)         Soda feldspar. (Na2O. Al2O3 6SiO2)

(iii)       Lime feldspar. (CaO. Al2O3. 6SiO2).

3. SAND

Sand or flint (SiO2) is the main constituent of ceramic industry. It is used as an opener for reducing the shrinkage of the clay. In addition to the principal raw materials, a wide variety of other minerals, salts and oxides are used as fluxing agents and special refractory ingredients. Some common fluxing agents which lower temperatures are borax, boric acid, soda ash, sodium nitrate, pearl ash, apatite, cryolite, iron, lead oxide etc

 

MANUFACTURE

All ceramic products are made by combining various amount of the raw materials such as clay, potter’s flint and feldspar etc. All these are mixed with water to form a paste called “slip”. The slip is passed through the filter press to remove almost all water and moist cake is obtained. The shape of the article is. designed by an’ artist, by potter wheel or by die pressing. The articles are now dried in air or chambers through warm air circulation. The dried articles are placed in clay boxes called “suggers” which are placed in the furnaces and firing kiln, called hovel oven so that the articles may not come in direct contact of the flame. The temperature of the first firing is as low as 700 °C and then as high as 2000°C.The objects obtained from the kiln in solid form is called “biscuits”. In order to make the earth ware impervious to liquid, glazes are applied on it. Various methods are applied for glazing such as dipping, pouring, spraying, dusting, volatization etc. the biscuit is again fired in a furnace at a temperature 700-900C°. During the firing, the articles become smooth and shining in appearance.

COLOURING OF ARTICLES

The colouring agents such as cobalt oxide (blue), copper oxide (green or red), iron oxide (yellow, orange or red), MnO2, (violet) etc. are mixed in paste.

CHEMICAL CONVERSIONS

All ceramic produets are made from low temperature to high temperature

range. Such temperatures cause a number of reactions for,. the chemical conversions.

  1. Dehydration at 150 to 650C°.

  2. Calcinations at 600 to 900C°.

  3. Oxidation of ferrous iron and organic matter at 350 To 900C°.

  4. Silicate formation at 900C° and higher. 

Some of the initial chemical changes arc relatively simple such as calcinations of CaCO3 and the dehydrations and decompositions of kaolinite. Other reactions such as silicate formations are quite complex and change with temperature. The common element of all ceramic products is clay and therefore the chemical reactions which occur on heating clay are quite important. The first effect of the heat is to drive off the water of hydration; this occurs at about 600 to 650C° and absorbs much heat, leaving an amorphous mixture of alumina and silica.

Al2O3. 2SiO2 .2H2O →      Al2O3 + 2SiO2 + 2H2O

As heating is continued, the amorphous alumina changes at 940C° to crystalline form of alumina with the evolution of considerable heat. At a slightly upper temperature, beginning at about I 00C°, the silica. and alumina combine to form mullite (3Al2O3.2SiO2).At a still higher temperature, the remaining silica is converted into crystalline cristobalite. The basic reaction in the heating of clay is as follows:

3(Al2O3.2SiO2. 2H2O) →  3Al2O3. 2SiO2+4SiO2 + 6H20

Knolinite                                                   Mullite              Cristobalite

An actual Ceramic body contains many more ingredients than clay itself. Hence the chemical reactions are more involved are there will be other chemical species besides mullitc and cristobalite present in the final product. All ceramic bodies undergo a certain amount of vitrification, or glass formation, during heating and the degree of vitrification depends upon the relative amounts of refractory and fluxing oxides in the composition. the temperature and the te of heating.

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